For DSL you do need an internet ethernet card. This usually comes standard on most computers. Look at the back of your laptop/computer and you should see a place where you can plug in your DSL connection (it looks like a bigger telephone connection).
[You could also use DSL to setup a wifi (wireless internet) in your house if you buy a wireless router. ]
Usually the DSL company will give you a CD to install or will come and install your connection themselves.
DSL connections are usually unlimited (you can use it as much as you want) so that's strange they have a cap. MBS usually refers to megabytes per second ie the speed of your connection. a good connection is usually 3 to 7 mbs.
hope that helps... ws